Chlorine and other halogens are frequently generated by an electrolysis of a brine of a salt of the halogen. Generally the salt is one of an alkali metal and the halogen. Cells used for this electrolytic process are subjected to a harsh chemical environment. Caustic products being produced in the cell, the brine and the halogen being produced, together can cause a short service life for cell mechanical components. Particularly for electrodes such as the cell anode, where chlorine is evolved during electrolysis, service life can be troublesome.
While previously a relatively inert material for use in fabricating anodes for use in these cells was the subject of diligent search, more recently electrodes fabricated from passivating or so-called valve metals have found wide acceptance in the generation of halogen by electrolysis. These valve metals commonly are considered to be titanium, tantalum, tungsten, bismuth, aluminum, niobium, zirconium and mixtures of these metals. Generally, the valve metals tend to form a surface barrier layer when exposed to oxidants that tend to protect the valve metal from further damage. Often this barrier layer is not significantly electrically conductive.
One valve metal finding broad acceptance for fabricating electrodes for use in halogen generation cells is titanium. Titanium withstands corrosive effects of the cell environment well and is, at least relative to the other valve metals, suitably resistant to corrosive effects of halogen generating cells. Titanium offers relative availability and cost advantages as a metal for use in fabricating chlorine cell components. On an absolute scale, however, titanium remains relatively expensive, and where a large number of electrodes are required for use in a halogen generating plant, for example, the cost can be substantial.
Another drawback to titanium is the electrical conductivity of the metal, much less than copper, gold and silver and considerably less than baser metals such as iron and nickel. When titanium is used to fabricate particularly a reticulate electrode widely used for halogen production, considerable care is required to ensure an adequate electrical current distribution throughout the reticulate structure. An adequate current distribution can result in a relatively elevated power inefficiency due to resistance losses as electrical current passes through the reticulate structure. Where the reticulate structure is fabricated from a relatively conductive substrate having a coating of the valve metal for protection, considerable cost savings are available both in titanium and in attachment of the reticulate electrode to a current feeder used to distribute electrical current.
Past proposals have attempted to provide titanium coated conductive substrates for use as an electrode by electolytic deposition of titanium in an aqueous electrolyte. These coatings have generally been unsatisfactory where used in a halogen generating electrolysis cell. Contamination of the titanium coating, non-uniform thickness, relatively poor adhesion to the substrate, and small crystal size are among explanations offered for failures of these titanium coatings giving rise to the dissatisfaction.
In another past proposal, pressure cladding of the substrate with titanium was proposed to provide an effective coating. Cladding is the application of one metal to the surface of another using, generally, pressure to create an interdiffused zone between metal at the surface of the substrate and the coating metal. This interdiffused zone includes one or more alloys of the substrate metal and the cladding metal, the metals intertwined in a progression of crystal states corresponding to progressive changes in composition through the zone. This interdiffusion effect can strengthen bonding between the substrate and cladding metal promoting adherence.
In forming reticulate type electrodes, particularly pressure cladding can produce a less than satisfactory result. Particularly at corners or edges of a mesh screening used for reticulate type electrodes, this pressure cladding technique can produce less than a satisfactorily integrated coating. Where pores, or other irregularities are present in a coating, attack on the substrate by contents of the electrolytic cell can quickly cause spalling of coating around the irregularity leading to rapid electrode failure.
Techniques are known for the formation of interdiffused coatings upon a substrate using electrodeposition from a fused salt electrolysis bath such as "Flinak" an eutectic mixture of lithium, sodium and potassium fluoride having a melting point of about 454.degree. C. Such techniques are shown and described in U.S. Pat. No. 3,479,159, French 1st Publication (Brevet) No. 2,075,857, and in 221 Scientific American 38 (1969). These references generally describe methods for forming solid solutions of coating metal on the substrate and intermetallics of the coating and substrate, but do not describe desirably adequate techniques for providing a relatively pure coating of a first metal on a relatively pure second metal substrate with one or more interdiffused zones between the relatively pure metals.
A solid solution is a homogeneous crystalline phase composed of at least two distinct chemical species occupying lattice or interlattice points within a crystalline structure at random. These solutions, for a given species pair, can exist in a range of species concentration.
Intermetallic compounds, also known by the terms Hume-Rothery or electron compounds, are alloys of usually two metals wherein a progressive change in composition of the alloy is accompanied by a progression of phases, each phase, generally differing in crystalline structure.
Generally in these solid solution or intermetallic compounds, hereinafter called alloys or interdiffused alloys for convenience, one of the component metals is possessed of a somewhat greater activity as manifested by a corresponding activity coefficient than the other metal. This more active metal is generally applied second.
However, in an electrolytic cell, the composition of any solid solution at the surface of an electrode coating contacting the cell environment can be critical. Where this coating surface includes a significant substrate metal content as an interdiffused alloy at the surface, the electrode may enjoy only a foreshortened lifespan in the corrosive cell environment. Conversely, where the coating surface can be maintained substantially free of the substrate metal, electrode life spans are less likely to be negatively influenced by the presence of the substrate metal.